[XrdCl/XrdSys] use custom semaphores only for glibc<2.21

- reorder pthread cleanap and setcanceltype statements in order to avoid
  a race condition
- use custom semaphores only for glibc<2.21 (in glibc>=2.21 the
  semaphore problem has been fixed)

src/XrdCl/XrdClUglyHacks.hh

@@ -28,9 +28,15 @@
 #include "XrdSys/XrdSysLinuxSemaphore.hh"
 #include "XrdSys/XrdSysPthread.hh"
 
+#if defined(__GLIBC__) && defined(__GLIBC_MINOR__)
+#define USE_LIBC_SEMAPHORE (__GLIBC__ >= 2 && __GLIBC_MINOR__ >= 21)  // in libc >= 2.21 the semaphore problem has been fixed
+#else
+#define USE_LIBC_SEMAPHORE 0
+#endif
+
 namespace XrdCl
 {
-#if defined(__linux__) && defined(HAVE_ATOMICS)
+#if defined(__linux__) && defined(HAVE_ATOMICS) && !USE_LIBC_SEMAPHORE
   typedef XrdSys::LinuxSemaphore Semaphore;
 #else
   typedef XrdSysSemaphore Semaphore;

src/XrdSys/XrdSysLinuxSemaphore.hh

@@ -155,13 +155,13 @@ namespace XrdSys
             {
               int r = 0;
 
-              pthread_setcanceltype( PTHREAD_CANCEL_ASYNCHRONOUS, 0 );
               pthread_cleanup_push( Cleanup, pValue );
+              pthread_setcanceltype( PTHREAD_CANCEL_ASYNCHRONOUS, 0 );
 
               r = syscall( SYS_futex, pValue, FUTEX_WAIT, newVal, 0, 0, 0 );
 
-              pthread_cleanup_pop( 0 );
               pthread_setcanceltype( PTHREAD_CANCEL_DEFERRED, 0 );
+              pthread_cleanup_pop( 0 );
 
               if( r == 0 )               // we've been woken up
                 break;